1. Field of the Invention
The present invention relates to a technology for fixing an indefinite potential generated at a radiation detection unit in which a plurality of photoelectric conversion units for converting radiation signals into charge signals is located.
2. Description of the Related Art
In recent years, in the field of a digital radiation imaging apparatus, there has been popularized a large-area flat panel type sensor of a same size optical system that uses, in place of an image intensifier, a photoelectric conversion unit to improve resolution, reduce a volume, and suppress distortion of an image. Imaging apparatuses using photoelectric conversion units include an amorphous silicon type, a charge-coupled device (CCD) type, and a complementary metal-oxide semiconductor (CMOS) type.
In the case of an image sensor that uses an amorphous silicon semiconductor on a glass substrate, a large-screen type can be easily manufactured. However, semiconductor characteristics are not sufficient for a high-speed operation. Microfabrication of an amorphous silicon semiconductor substrate on the glass substrate is more difficult than a single-crystal silicon semiconductor substrate. As a result, a capacity of an output signal line becomes large, generating kTC noise.
The CCD imaging apparatus is not suited to achievement of a large screen while it is a complete depletion type and high in sensitivity. An area of the CCD becomes large because it is a charge transfer type. When the number of charge transfer stages increases, driving voltages are different between a drive end and the vicinity of a center, causing a difficulty of complete transfer. For power consumption CVf2 (C: capacity between substrate and well, V: pulse amplitude, and f: pulse frequency), C and V are larger as an area is larger, and power consumption is ten times as large as that of a CMOS image sensor.
Japanese Patent Application Laid-Open No. 2002-344809 discusses a large-screen flat panel type sensor where a CMOS image sensor is used for a photoelectric conversion unit, and a large area is achieved by tiling a rectangular image sensor formed by cutting out a CMOS photoelectric conversion unit from a silicon semiconductor wafer into a rectangular shape. Microfabrication of the CMOS image sensor enables reading faster than the amorphous silicon and acquisition of higher sensitivity. A large area can be easily achieved without any problems in number of charge transfer stages or power consumption of the CCD image sensor. Thus, it is known that the CMOS image sensor is advantageous for the large-screen flat panel type sensor particularly as a moving image imaging apparatus.
However, in the CMOS image sensor that simultaneously achieves pixel addition and sensitivity switching, when driven in a high-sensitivity mode, an indefinite potential is generated in a circuit of the CMOS image sensor. During moving image capturing, when there is an indefinite potential in the circuit of the CMOS image sensor, a minute amount of leakage becomes indefinite between a gate and a source of a metal-oxide semiconductor (MOS) transistor in the circuit of the CMOS image sensor, affecting the operation as random noise of each frame.
In the CMOS image sensor, dark current is generated during a period where it is not irradiated with light for capturing. Thus, the CMOS image sensor has an offset value, and each pixel outputs a value that is not zero as a optical signal even when the CMOS image sensor is not irradiated with light. There is a method for setting optical signal data acquired without being irradiated with light as a fixed pattern noise (FPN) pattern, and subtracting the FPN pattern from the optical signal data acquired during capturing of the moving image. However, a potential at a floating portion for each moving image capturing changes with time. Hence, there is a difference between a potential at a floating portion in the CMOS image sensor, which is acquired before the FPN pattern is captured, and a potential at a floating portion when a moving image is actually captured. This causes a difference in noise components generated due to the indefinite potential at the floating portion between the FPN pattern and moving image data for FPN correction. As a result, correct FPN correction cannot be carried out.